I bought a 5D after trying a colleague's camera. The reason initially was the very low price and excellent performance of the 5Dc.

Then I fall in love with the 5D color rendering, the capacity to use the lenses with their real focal length, the better control of DOF for portraiture, the better image quality when printing, better focusing system than original k5 (focus point is too large in the original k5 and is almost an outdated technology).

I was considering for a while the k5-iis, but for an additional few hundred dollars I got a 6D. Now that I have several FF Canon lenses and that I have been using a FF system, I have a clear road map of where I want to go in terms of camera/lenses.